1. Technical Field
The present disclosure relates to a method for controlling a switching regulator.
The present disclosure also relates to a control circuit for a switching regulator.
The disclosure concerns in particular, but not exclusively, a method and a circuit for controlling a switching regulator of the quasi-resonant flyback type and the following description is made with reference to this specific field of application with the sole purpose of simplifying its presentation.
2. Description of the Related Art
Switching regulators are used in power supply systems to provide a constant voltage or current in output as the supply voltage varies and as the load applied to the output terminals varies. The quasi-resonant (QR) flyback switching regulator is a regulator with an insulation arranged between the input terminals and the output terminals made from a magnetizable part, for example a transformer, which is magnetized and demagnetized through a switch. The switch is made through a switching transistor of the bipolar, MOSFET or similar type.
An example of a QR flyback regulator 1 is shown in FIG. 1 whereas FIG. 2 shows the relative waveforms. During the closed time of the switch 5, Ton, there is an accumulation of energy in the transformer 2 and during the open time of the switch, Toff, with the demagnetization of the transformer, the energy stored in the transformer is supplied to the output circuit to constantly feed the load. A secondary demagnetization current Is is supplied to the output circuit through the secondary winding LS of the transformer 2.
A control circuit 10 detects, directly or through a sensor, the demagnetization state of the transformer 2, and activates the switch 5 when successful demagnetization is detected.
In the switching regulator of FIG. 1, the control circuit 10 detects the demagnetization through a sensor defined by an auxiliary winding LAUX of the transformer 2. It should be noted that the oscillation of the voltage Vaux on the auxiliary winding corresponds to the oscillation of the voltage VDS through the switch 5 scaled by the ratio of the coils between the primary winding LP and the auxiliary winding LAUX.
The control circuit 10 comprises a Zero-Current Detection or Zero-Crossing Detection (ZCD) circuit 12, which has a terminal associated, through a resistance RZCD, with the auxiliary winding LAUX of the transformer 2. When the voltage Vaux on the auxiliary winding detected by the terminal, on the negative-going edge, is less than a trigger threshold VZCDtrig, the ZCD circuit 12 generates a trigger signal that sets the PWM latch 15 causing the switch 5 to be switched on. In order to “arm” the ZCD circuit 12 before detecting the trigger threshold VZCDtrig, the terminal detects a voltage Vaux on the rising edge that exceeds a first threshold VZCDarm of greater value than the trigger threshold VZCDtrig, as indicated in the waveform of FIG. 2.
With a trigger voltage VZCDtrig fixed at a value close to zero, the demagnetization of the transformer 2 is detected when the voltage on the auxiliary winding LAUX is zero whereas the voltage on the primary winding LP, which corresponds to the voltage VDS that passes through the switch 5, is equal to the input voltage Vin.
During the open time Toff of the switch 5, the voltage VDS stabilizes, after an initial transient, at a practically constant reference value (Vr+Vin), whereas the secondary demagnetization current Is is supplied to the output circuit.
As highlighted in FIG. 3, due to the presence of parasitic components in the circuit 1, after demagnetization when the secondary demagnetization current Is goes to zero, the voltage VDS takes on a damped sinusoidal oscillation the waveform of which has a period equal to TR with relative maximum values and relative minimum values.
Based on these properties, the voltage VDS, i.e., its image Vaux obtained at the ends of the auxiliary winding LAUX, can be taken as recirculation signal, i.e., signal capable of highlighting when the energy stored in the transformer during the accumulation time Ton is recirculating (Is>0) and when said recirculation ends (Is=0).
In order to reduce the power losses to the minimum, during the switching of the switch 5, operation of the “valley-switching” type is foreseen, in which the switch is activated at a valley of the damped oscillation of the voltage VDS.
Through a time delay portion of the ZCD circuit 12, the switch is activated at the first valley of the voltage VDS that occurs after a delay equal to ¼ the period TR after demagnetization when the secondary demagnetization current Is goes to zero.
With respect to the standard flyback regulator which has a fixed switching frequency, the QR flyback regulator has some advantages, like a low presence of Electro-Magnetic Interference (EMI) and better safety in short-circuit conditions.
However, the QR flyback regulator 1, in the presence of a variation of the input voltage or a variation of the load, varies the switching frequency.
For example, in the case of a reduction of the load, the frequency tends to increase and, as known, the power losses are proportional to the switching frequency of the switch. This leads to a considerable reduction in efficiency of conversion at low loads.
Different solutions are known for reducing the power losses to a minimum in a QR flyback regulator that operates with variable switching frequency.
A solution known as “frequency foldback” or “valley-skipping” foresees to keep the switch 5 in the interdicted state for a time longer than or equal to a waiting time TBLANK, i.e. of making the activation impulses generated by the ZCD circuit 12 during the waiting time TBLANK substantially “invisible”.
In other words, as shown in FIG. 3, a window F having a duration equal to the waiting time TBLANK, is activated upon the deactivation of the switch 5. All of the activation impulses generated by the control circuit are ignored if they fall within the window F, whereas the first activation impulse generated after the waiting time TBLANK makes the switch conduct.
The delay in the activation of the switch with respect to the demagnetization of the transformer can be calculated as:
      Td    k    =                              2          ⁢          k                -        1            2        ⁢          T      R      
where: k=1,2 . . . the number of valleys                TR is the period of the damped sinusoidal oscillation of the voltage VDS after demagnetization of the transformer.        
As shown in FIG. 4a, it is advantageous to modulate the waiting time TBLANK as a function of a control voltage VCSref referred to the current detected between the switch and the ground reference voltage VGND. In particular, with a reduction in the output load there is a reduction of the control voltage VCSref and a lengthening of the waiting time TBLANK.
As shown, on the other hand, in the diagram of FIG. 4b, the switching frequency gradually falls with the increase in input power Pin and based on the number of valleys that are skipped.
During valley-skipping operation it is possible to observe some irregular switching cycles, as shown in FIG. 5, in which the switching takes place with an alternation between the ignored valleys, which identifies a phenomenon called “valley-jump”, in which the switching takes place in some cases in the presence of the second valley of the damped sinusoidal oscillation of the voltage VDS and in other cases in the presence of the third valley.
Such a “valley-jump” phenomenon is due to the fact that the switch switches at a discretized value TdK, the window F has a value that is fixed as a function of the control voltage VCSref, whereas for energy balance the switching should take place in an intermediate point between two contiguous cycles.
The valley jump phenomenon introduces a low-frequency component that creates perturbations in the primary current of the switch and induces an audible noise generated by mechanical vibrations due to the presence of the magnetic components.
The low-frequency noise is a drawback that is more serious in the case in which the switching regulators are contained in portable electronic devices.
Manufacturers of switching regulators have defined, as an internal specification for low-frequency noise, a maximum threshold of 25 dB(A)/20.0 μPa measured at a distance of 5 cm.
Some solutions have been proposed to avoid the occurrence of noise in switching regulators.
One solution is described in United States patent application No. US2011/0182088 to Lidak et al. and it foresees an internal up/down counter the operation of which is schematically shown in FIG. 6 with a number of valleys of between 1 and 4. It is foreseen for there to be hysteresis behavior as a function of the control voltage VCSref of the circuit.
In particular, with a reduction of the output load, which corresponds to a reduction of the control voltage VCSref, there is an increase in the number of valley from 1 to 4, whereas with an increase of the output load, and a consequent increase of the control voltage VCSref, there is a reduction of the valley down to 1.
Substantially, the control circuit is interdicted at a selected number of valleys until a significant change in the output load.
A further solution is shown in FIG. 7 and it is made by the company Infineon and inserted in the QR Flyback controller known by the reference identification ICE2QS03x.
The solution foresees the presence of two counters, with the first up/down counter that indicates the number of valleys that are to be skipped, whereas the second counter detects the number of valleys after the demagnetization of the transformer, by activating the switch when the number of the second counter equals the number of the first counter.
The counting direction of the first counter depends on the control voltage VCSref that is detected in sampling intervals based on an internal clock, which in the case indicated is equal to T=48 ms. A voltage band BV is identified between a low first value VFBZL and a high second value VFBZH.
When the sampled value of the control voltage VCSref is within the band BV the first counter stays unchanged, and when the sampled value is below the first value VFBZL the first counter is increased by one, but if it is higher than the second value VFBZH the first counter is decreased by one, and if the value of the control voltage VCSref is greater than a third maximum threshold VFBR1, the first counter is automatically brought to one.
The height of the band BV is calculated so that the counter remains unchanged with a constant output load. Therefore, the activation of the switch takes place in the presence of the number of valleys of the second counter until the output load changes significantly.